Fix documentation of spring.kafka.producer.batch-size

Closes gh-13127

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/kafka/KafkaProperties.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -444,7 +444,8 @@ public class KafkaProperties {
 		private String acks;
 
 		/**
-		 * Number of records to batch before sending.
+		 * Default batch size in bytes. A small batch size will make batching less common
+		 * and may reduce throughput (a batch size of zero disables batching entirely).
 		 */
 		private Integer batchSize;
 

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -917,7 +917,7 @@ content into your application; rather pick only the properties that you need.
 	spring.kafka.listener.concurrency= # Number of threads to run in the listener containers.
 	spring.kafka.listener.poll-timeout= # Timeout in milliseconds to use when polling the consumer.
 	spring.kafka.producer.acks= # Number of acknowledgments the producer requires the leader to have received before considering a request complete.
-	spring.kafka.producer.batch-size= # Number of records to batch before sending.
+	spring.kafka.producer.batch-size= # Default batch size in bytes.
 	spring.kafka.producer.bootstrap-servers= # Comma-delimited list of host:port pairs to use for establishing the initial connection to the Kafka cluster.
 	spring.kafka.producer.buffer-memory= # Total bytes of memory the producer can use to buffer records waiting to be sent to the server.
 	spring.kafka.producer.client-id= # Id to pass to the server when making requests; used for server-side logging.